"Our students will sit the test and leave Catholic schools. People say 'why don't we have our own?', but we're not interested in increasing that inequity," Mr Whitby said.

"It forces us to provide learning that's fit for each student and it ultimately makes a statement about the type of society we want to build.

"We need to build a robust learning community and invest in building that capacity in everyone, not just an elite group of schools."

Mr Whitby said he echoes concerns raised by NSW Education Minister Rob Stokes and Department of Education secretary Mark Scott that the growing tutoring industry built around the selective schools test is making places more easily accessible for students from wealthier families.

"The minister has raised this, parents will try to buy their way into these schools," said Mr Whitby, who was awarded the prestigious Sir Harold Wyndham Medal Award last year for outstanding contribution to the education of young people in NSW.

"We know how important education is and every child needs to have the same opportunity and access.

"Having selective schools props up a system where those from a more well-off path are more likely to have access [to better resources]."

"We need to have public schools that are inclusive of everyone rather than deliberately separate children on the basis that some are gifted and talented and others are not," Mr Stokes said.

Nearly 15,000 year 6 students sat the selective schools entrance test in March this year, competing for 4226 places across NSW's 19 fully-selective and 29 partially selective schools.

Selective schools are also opposed by the NSW Teachers Federation, with president Maurie Mulheron saying the system segregates students without adding any significant educational value.

"They don't add anything to a child's education but they do damage to school communities by taking them out of comprehensive schools, fragmenting the system and creating social segregation," Mr Mulheron said.

"The best kids would be successful anywhere and many suffer from going from being the best in their primary school to being at the bottom of their selective high school.

"And they deprive comprehensive school of the brightest kids, so you struggle to run the extension class or the physics class and the remaining bright kids suffer in that way."